Fall 2012 Statistics-
SLC: 45% of applicants admitted
Bennington: 62% of applicants admitted
Hampshire: 51% of applicant admitted
Those statistics are ever-changing. I know just a couple of years ago, SLC was at like 58% acceptance. But anywho, statistics like that don't really do justice to SLC's selection process. It's highly individualized - based moe on your essay, gpa, and extra curriculars than tests or numbers. So, if you're interested, don't let low admissions rates deter you.
I don't know much about Hampshire at all... But Bennington and Sarah Lawrence are comparable. SLC is a small school at about 1300 undergrads, and Bennington is half the size of that. Academically they are very similar - self-designed academic curricula, small class sizes. Sarah Lawrence is unique in their conference structure - one on one meetings with professors and independent and class-oriented work. Bennington, on the other hand, allows you to design your own major, wheras at Sarah Lawrence, you graduate with a degree in Liberal Arts.
Socially, they're similar as far as I know - liberal, hippie, artsy schools
Environmentally, SLC is in a small suburban setting close to NYC whereas Bennington is a more rural setting.
They are very similar overall and the differences are best ascertained by visits to both.
Best of luck with college applications!